1. Field of the Invention
The present invention relates to an optical scanning apparatus which forms a latent image by scanning on an image bearing member (e.g., a photosensitive drum) with a plurality of laser beams modulated by an image signal and, more particularly, to an optical scanning apparatus for controlling the interval between main scanning lines formed by a plurality of semiconductor lasers and an image forming apparatus which incorporates the optical scanning apparatus.
2. Description of the Related Art
There is available an image forming apparatus which forms a latent image by modulating laser beams emitted from a plurality of semiconductor lasers with an image signal and raster-scanning the laser beams on an image bearing member using a scanner motor. Such a multi-beam image forming apparatus scans a plurality of raster lines (scanning lines) in parallel with a plurality of laser beams emitted from a plurality of laser devices. When an optical unit which synthesizes a plurality of laser beams deforms due to a temperature variation, the interval between a plurality of laser beams (to be referred to as a beam interval) on an image bearing member changes. As a consequence, the interval between scanning lines on the image bearing member which are formed by these beams also changes. A beam interval variation occurs independently of a sub-scanning amount (to be referred to as a sub-scanning pitch) per main scanning which is provided by the rotation of the photosensitive drum. For this reason, unevenness occurs in the interval between scanning lines (to be referred to as a scanning line interval) on the photosensitive drum which are formed by different scans. For example, as the beam interval increases, the interval between the scanning lines formed by the beams increases. In contrast to this, the interval between the lowermost scanning line of a plurality of scanning lines in one scan and the uppermost scanning line of a plurality of scanning lines in the immediately succeeding scan decreases. Note that scanning line interval unevenness will be referred to as line pitch unevenness.
Under the circumstances, there has been proposed a technique of suppressing a beam interval variation due to thermal expansion by forming a holder, on which laser devices are mounted, and an optical box, in which a rotating polygon mirror, an optical lens, and the like are mounted, by using materials having almost the same thermal expansion coefficient (see, for example, patent reference 1). There has also been proposed a method of finely adjusting a beam interval by using a prism for adjustment (see, for example, patent reference 2).
There has also been proposed an apparatus including an optical system (optical unit synthesizing motor) which can automatically adjust a beam interval when converting a plurality of laser beams into parallel light beams at a predetermined interval. This method uses a beam interval sensor to detect the beam interval between a plurality of applied laser beams. A motor rotation amount calculation circuit determines the rotation amount of the optical unit synthesizing motor on the basis of the difference between the beam interval detected by the beam interval sensor and a predetermined value (e.g., with 1,200 dpi, d=21.17 [μm]), thereby controlling the optical unit synthesizing motor to make the beam interval become a predetermined value.
[Patent Reference 1] Japanese Patent Laid-Open No. 11-202232
[Patent Reference 2] Japanese Patent Laid-Open No. 8-68956
The above conventional techniques cannot, however, correct a sub-scanning pitch unevenness caused by the rotation unevenness of the image bearing member or driving motor (transfer belt motor or drum motor). For this reason, the interval between scanning lines formed by one laser beam varies in accordance with sub-scanning pitch unevenness, and the scanning line interval varies for every number of beams forming multi-beams. That is, scanning line interval unevenness occurs.
In addition, the linear velocity of the surface of an image bearing member varies due to gradual wear and tear caused by the long-term use of the member, and the moving distance of the image bearing member in the sub-scanning direction varies even if it is driven/controlled with constant rotation. With this variation, the sub-scanning pitch changes over time, and unevenness occurs.